Heretofore, an energy storage device described in JP-A-11-250909 is known, for example. This energy storage device includes a positive electrode, and a negative electrode capable of absorbing/desorbing lithium ions. The negative electrode has a negative active material containing a composite carbon material including graphite and non-graphitizable carbon.
The graphite has a high capacity and is excellent in potential flatness; however, absorption/desorption performance of lithium ions is deteriorated when repeating charge-discharge. On the other hand, the non-graphitizable carbon has a smaller capacity and a lower power potential at a low SOC region than the graphite; however, absorption/desorption performance of lithium ions is hardly deteriorated when repeating charge-discharge.
In a conventional technology, it is said that a secondary battery having a large capacity and excellent cycle performance can be obtained by employing a constitution in which the negative electrode includes graphite and non-graphitizable carbon.
However, it is difficult to adequately improve the input performance of the secondary battery even by the conventional technology, and so it has been thought to improve input performance by reducing a particle size of a negative active material.
However, the cycle performance may be deteriorated when the particle size of the graphite and the particle size of the non-graphitizable carbon are simply decreased.